1. Technical Field
This invention relates to the art of interfacing metals having differing thermal expansion characteristics and, more particularly, to the art of compensating for such differences in thermal expansion when such joined metals are used at widely varying temperature conditions.
2. Description of the Prior Art
In modern internal combustion engines, dissimilar metal interfaces have included both rigid and moving interfaces. A rigid interface exists between an aluminum head and an iron-based cylinder block, joined by steel bolts with a gasket interposed therebetween. Similarly, a rigid interface may exist between an aluminum cylinder block and cast-iron main bearing caps attached to the block by steel bolts. The coefficient of thermal expansion of the steel bolts, or of the cast-iron part, relative to the aluminum part, differs considerably and may result in widely differing clamping forces between the parts. The interface can loosen under some temperature conditions that may cause engine performance problems.
Attempts have been made to use gasket materials with excessively low relaxation rates at the rigid head-to-block interface and thereby maintain a more uniform level of bolt forces (see SAE article "Aluminum Engines Require Special Gasketing Techniques", Vol. 91, No. 4, pp. 48-50, 1983). Such techniques are expensive and not totally satisfactory to eliminate all leakage. Other attempts to improve rigid interfaces have included the use of inserts having high resistance to creep which have been cast-in-place in an aluminum head to avoid plastic deformation of the head under the stress of steel bolts (see U.S. Pat. No. 4,450,800). The insert in this disclosure is comprised of nickel and iron providing a coefficient of thermal expansion the same as the aluminum head but with greater pressure resistance than aluminum. This, in no way, modifies the thermal expansion characteristic of either interfacing part, thus resulting in the same propensity to looseness.
Moving interfaces exist between rotating members and their bearings or between a reciprocating piston and its bore walls. At the interface between an iron crankshaft and an aluminum bearing support, the prior art has attempted to modify the absorption of impact stress at such interface to eliminate gradual distortion of the support. In U.S. Pat. No. 3,089,735, a bearing support insert is used to provide a greater outer surface area than inner surface area. In this disclosure, a cast-in-place insert comprised of aluminum or magnesium is kneaded to have high compression resistance; it is preferably bonded during casting of the block by use of lead, cadmium, tin or zinc. No attempt is made to substantially modify the thermal expansion characteristic of the block at the interface by material substitution.
Although metal matrix composites are known which change the physical characteristics of the metal matrix by introducing a ceramic phase, little application research has been carried out to adapt such technology to dissimilar metal interfaces which may be employed in an internal combustion engine.
Accordingly, the primary goal of this invention is (i) to modify the thermal expansion characteristic of at least one part of an interface between dissimilar metals to make them dimensionally compatible at varying temperature conditions, and (ii) to do so by cast-in-place techniques that avoid the buildup of internal stresses that lead to breakage or delamination.